Values.SendMessageContentBlockStartEventSourceEvent emitted when a new content block starts
type nonrec t = {index : Integer.t option;Zero-based index of the content block
*)type_ : String_.t option;The type of content in this block
*)id : String_.t option;Block identifier
*)parentId : String_.t option;Optional parent block ID for nested content blocks (e.g. subagent tool calls)
*)sequenceNumber : Integer.t option;Event sequence number
*)}